Trajectory tracking with four-wheel steering and steering limits

ABSTRACT

Four-wheel steering of a vehicle, e.g., in which leading wheels and trailing wheels are steered independently of each other, can provide improved maneuverability and stability. A first vehicle model may be used to determine trajectories for execution by a vehicle equipped with four-wheel steering. A second vehicle model may be used to control the vehicle relative to the determined trajectories. For instance, the second vehicle model can determine leading wheels steering angles for steering leading wheels of the vehicle and trailing wheels steering angles for steering trailing wheels of the vehicle, independently of the leading wheels.

BACKGROUND

Some autonomous vehicle systems provide for independent steering, e.g.,of front and rear wheels. More specifically, unlike many conventionalvehicles that include steerable front wheels and fixed rear wheels, somevehicles are intended for driving in either of two “forward” directions,e.g., such that wheels at a first end can be leading or trailing wheels.To enable this bidirectionality, the wheels at each end areindependently steerable. Many conventional autonomous systems of thistype maneuver the vehicle by steering the leading wheels and maintainingthe trailing wheels fixed. Accordingly, despite the four-wheel steeringfunctionality, the vehicle performs as a conventional, front-wheelsteering vehicle. Approximating a two-wheel steering vehicle may be thepreferred approach, because modeling two-wheel vehicles may be simplerand relatively well-known. However, controlling the autonomous vehiclewith active steering at all four wheels may provide benefits over atwo-wheel steering design. For instance, four-wheel steering may providegreater maneuverability and/or stability than conventional two-wheelsteering systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentify the figure in which the reference number first appears. Thesame reference numbers in different figures indicate similar oridentical items.

FIG. 1 includes a perspective view of an example vehicle and a schematicdepiction of a portion of an example four-wheel steering system,according to aspects of this disclosure.

FIG. 2 is a schematic top view of an example vehicle including anexample steering control system, according to aspects of thisdisclosure.

FIG. 3 is a textual and visual flow diagram of an example sequence ofdetermining trajectories based on a four-wheel steering model of avehicle, according to aspects of this disclosure.

FIG. 4 is a textual and visual flow diagram of an example sequence ofcontrolling a vehicle to follow a trajectory using four-wheel steering,according to aspects of this disclosure.

FIG. 5 is a textual and visual flow diagram of an example sequence fordetermining steering commands with limit steering constraints, accordingto aspects of this disclosure.

FIG. 6 is a block diagram of an example system for implementingfour-wheel steering and related techniques, according to aspects of thisdisclosure.

FIG. 7 is a flow diagram of an example process for controlling a vehicleusing four-wheel steering, according to aspects of this disclosure.

DETAILED DESCRIPTION

This disclosure is generally directed to systems and techniques forcontrolling steering for a vehicle. In some examples, the techniquesdiscussed herein may include controlling steering for a first subset ofwheels of the vehicle according to first controls and controllingsteering for a second subset of wheels of the vehicle according tosecond controls. Specifically, and in some instances regardless of anorientation of the vehicle, the leading wheel can be steered accordingto a first steering angle and the trailing wheels can be steeredaccording to a second steering angle, e.g., independently of the firststeering angle.

Examples of this disclosure include one or more vehicle computingsystems executing a planning component to determining one or moretrajectories for the four-wheel steering capable vehicle. For instance,the planning component may determine trajectories using a kinematicvehicle model. The kinematic vehicle model may assume zero lateralvelocity, that is, no sideslip for the vehicle, at a point representingthe vehicle. The point may be equidistant from a first axle through theleading wheels of the vehicle and a second axle through the trailingwheels of the vehicle. Using this kinematic vehicle model, the planningcomponent may determine a first steering angle for leading wheels of thevehicle to navigate to a destination, e.g., based on a current state ofthe vehicle. In examples, the planning component can determine a seriesof vehicle states, including leading wheel steering angles, as atrajectory. The kinematic vehicle model assumes zero lateral velocity,and thus implicitly that the trailing steering angle is the additiveinverse of the leading steering angle. Stated differently, the kinematicvehicle model assumes mirrored steering for the vehicle, e.g., about alateral axis through the point representing the vehicle.

Also in aspects of this disclosure, the vehicle computing system(s) cancause the vehicle to execute the trajectories determined by the planningcomponent. For instance, the vehicle computing system(s), e.g.,executing a tracking system, can generate one or more of feedforwardsteering angles, feedback steering angles, and limit steering angles todetermine a leading wheel steering angle and a trailing wheel steeringangle to execute the trajectories. In implementations, the vehicleincludes a first steering controller for controlling steering of theleading wheels, e.g., according to the leading wheel steering angle, anda second steering controller for controlling steering of the trailingwheels, e.g., according to the trailing wheel steering angle.

In aspects of this disclosure, the tracking system can determine theleading and trailing wheel steering angles based at least in part on adynamic model of the vehicle. Thus, while the planning component may usea first, kinematic model of the vehicle to determine trajectories, thetracking system may use a second, dynamic model of the vehicle tocontrol the vehicle to follow the trajectories. Like the first model,the dynamic model may also assume zero lateral velocity, or zerosideslip of the vehicle to determine the leading and trailing steeringangles. In examples, the tracking component, executing the dynamicmodel, may determine a feedforward command based on a current state ofthe vehicle relative to a state determined by the trajectory. Thetracking component may also determine a feedback command based at leastin part on a tracking error of the vehicle, e.g., one or moredifferences between a current state of the vehicle and the trajectory.In at least some examples, the tracking error includes a lateral offsetand/or a heading error. The dynamic model may determine the leadingsteering angle and the trailing steering angle as steering angles toeliminate the tracking error.

In some instances, the leading and trailing steering angles may also bebased at least in part on a ratio between the trailing steering angleand the leading steering angle. For example, at relatively lowervelocities, a ratio of the trailing steering angle to the leadingsteering angle may be negative. At lower speeds, the negative ratio mayprovide greater maneuverability. At relatively higher velocities, aratio of the trailing steering angle to the leading steering angle maybe positive. At high speeds, the positive ratio may provide greaterstability.

Also in aspects of this disclosure, the tracking component can determinealtered leading and/or trailing steering angles, e.g., when controllingthe vehicle according to zero sideslip is not possible. For instance,the leading wheels may have a first maximum steering angle and thetrailing wheels may have a second maximum steering angle. When thetracking system generates leading and/or steering commands that exceedthe first and/or second maximum steering angle(s), such steering anglescannot be practicably executed. Accordingly, in some implementations,the tracking system may also generate one or more corrective terms toalter the determined leading and/or trailing steering angles. As notedabove, the tracking system may determine the leading and trailingsteering angles based on eliminating sideslip or zero lateral velocity.In contrast, the tracking system may ease this constraint when one orboth of the leading steering angle and/or the trailing steering anglewould otherwise exceed the respective first or second maximum steeringangle, to find the corrective steering term(s).

Systems and techniques described herein can provide many benefits. Forinstance, the four-wheel steering systems described herein can provideincreased functionality relative to two-wheel steering systems. Forinstance, at lower speeds four-wheel steering systems can providegreater maneuverability, e.g., by providing a higher effective turningradius. At relatively higher speeds, four-wheel steering systems canprovide great stability during steering. Because of these benefits, apassenger experience, including safety outcomes, can be improved. Thesystems and techniques can also be readily integrated into existingvehicles. For example, while existing autonomous vehicles may becontrolled using two-wheel steering, and thus the control infrastructureof such vehicles is developed for two-wheel steering, the systems andtechniques described herein can be readily integrated into theseexisting systems. For instance, because of the modeling techniques usedin aspects of this disclosure, the logic and programming associated withexisting systems can readily implement four-wheel steering. Otherfeatures and benefits are also apparent from the following description.

The techniques and systems described herein may be implemented in anumber of ways. Example implementations are provided below withreference to the figures.

FIG. 1 shows an example environment 100 through which an example vehicle102 is traveling. The example vehicle 102 may be a driverless vehicle,such as an autonomous vehicle configured to operate according to a Level5 classification issued by the U.S. National Highway Traffic SafetyAdministration, which describes a vehicle capable of performing allsafety-critical functions for the entire trip, with the driver (oroccupant) not being expected to control the vehicle at any time. In suchexamples, because the vehicle 102 may be configured to control allfunctions from start to completion of the trip, including all parkingfunctions, it may not include a driver and/or controls for driving thevehicle 102, such as a steering wheel, an acceleration pedal, and/or abrake pedal. This is merely an example, and the systems and methodsdescribed herein may be incorporated into any ground-borne, airborne, orwaterborne vehicle, including those ranging from vehicles that need tobe manually controlled by a driver at all times to those that arepartially- or fully-autonomous.

The example vehicle 102 may be any configuration of vehicle, such as,for example, a van, a sport utility vehicle, a cross-over vehicle, atruck, a bus, an agricultural vehicle, or a construction vehicle. Thevehicle 102 may be powered by one or more internal combustion engines,one or more electric motors, hydrogen power, any combination thereof,and/or any other suitable power sources. In the illustrated example, thevehicle 102 is generally travelling in the direction indicated by arrow104. Accordingly, a first end 106 of the vehicle 102 is a front orleading end, and an opposite, second end 108 of the vehicle 102 is arear or trailing end. The example vehicle also includes two leadingwheels 110 (of which only one is labelled) and two trailing wheels 112(of which only one is labelled. Although the example vehicle 102 hasfour wheels 104, the systems and methods described herein may beincorporated into vehicles having fewer or a greater number of wheelsand/or tires.

In implementations of this disclosure, the vehicle 102 has four-wheelsteering, such that both the leading wheels 110 and the trailing wheels112 are steerable independently, e.g., relative to each other and/orrelative to the vehicle 102. The four-wheel steering may facilitatebi-directionality, such that the vehicle 102 may operate generally withequal performance characteristics in all directions. For instance, asshown in FIG. 1, the first end 106 of the vehicle 102 is the front endof the vehicle 102 when travelling forward in the direction shown by thearrow 104, but the vehicle 102 may also travel forward in the oppositedirection, e.g., with the first end 106 as the rear end of the vehicle102. Similarly, the second end 108 of the vehicle 102 is the rear end ofthe vehicle 102 when travelling in the direction of the arrow 104, andthe second end 108 is the front end of the vehicle 102 when travellingforward in the direction that is the opposite of the arrow 104.

In some conventional vehicles that include four-wheel steering, thevehicle 102 may still be controlled generally as a vehicle withtwo-wheel steering. For instance, when the first end 106 is the front orleading end, as in FIG. 1, the leading wheels 110 may be steered,whereas the trailing wheels 112 are fixed. Similarly, when the secondend 108 is the leading end of the vehicle 102, the leading wheels 110may be fixed and the trailing wheels 112 may be steered. However,aspects of this disclosure involve steering both the leading wheels 110and the trailing wheels 112 to provide true four-wheel steering.Specifically, in the illustration of FIG. 1, both the leading wheels 110and the trailing wheels 112 are steering the vehicle 102 to maintain aheading along a path 114, e.g., to implement a lane-change. The path 114can be a visualization associated with a trajectory along which thevehicle 102 is to travel, as detailed further herein. In some instances,using four-wheel steering as shown in FIG. 1 can provide addedstability, e.g., at higher speeds. Four-wheel steering may alsofacilitate greater maneuverability, for example, by increasing aneffective turning radius of the vehicle 102 in small spaces or crowdedenvironments.

FIG. 1 also includes a block diagram of an example vehicle computingsystem 116 used to implement four-wheel steering at the vehicle 102. Forexample, the vehicle computing system 116 includes a planning component118, vehicle system controllers 120, and a tracking component 122. Forease of understanding, components of the vehicle computing system 116are shown and described as separate components, although aspects of theillustrated components may be shared and/or additional components may beincluded for carrying out functionality described herein and attributedto the illustrated components.

The planning component 118 is generally configured to generatetrajectories 124. For example, the planning component 118 of the vehicle100 may include instructions stored on a memory that, when executed byprocessor(s), configure the processor(s) to generate data representativeof one or more trajectories of the vehicle 102. The planning component118 can receive data representing a location of the autonomous vehicle100 in its environment and other data, such as local pose data, and theplanning component 118 can use such data to generate the trajectories124. Individual of the trajectories 124 may include a position or poseto which the planning component 118 instructs the vehicle 102 to move,e.g., along the path 114. For instance, the trajectories 124 can includeone or more vehicle states at one or more locations along the path 114.In some examples, the planning component 118 may also be configured todetermine projected positions and/or poses, e.g., as additionaltrajectories, predicted to be executed by the autonomous vehicle 102. Insome examples, the planning component 118 may substantially continuously(e.g., every 1 or 2 milliseconds) generate a plurality of potentialtrajectories with which to control the autonomous vehicle 102 and mayselect the trajectories 124 from these candidate trajectories. Theselection may be based at least in part on a desired destination, acurrent route, a drive mode, a current vehicle trajectory, and/or objecttrajectory data (e.g., of one or more objects in the environment 100).Upon generating and/or selecting the trajectories 124, the planningcomponent 116 may transmit the trajectories 124 to the vehicle systemcontrollers 120, e.g., as a reference trajectory command to control theautonomous vehicle 102.

As illustrated in FIG. 1, the planning component 118 includes a firstvehicle model 126, and the first vehicle model 126 is used to determinethe trajectories 124. In examples, the first vehicle model 126 is akinematic model of the autonomous vehicle 102. The kinematic model maybe chosen for its relative simplicity, which allows for the planningcomponent 118 to generate the trajectories 124 at a relatively highfrequency and with relatively reduced computational resources. Asdetailed further herein, the first vehicle model 126 may be based on anassumption that the vehicle has a single point of zero lateral velocity.In conventional two-wheel steering vehicles, a point centered betweenthe trailing wheels on an axis extending between the axes of rotation ofthe trailing wheels may be the point of zero lateral velocity. However,in a four-wheel steering vehicle, such as the vehicle 102, because thetrailing wheels 112 also are steerable, the point centered therebetweendoes not have zero lateral velocity. Instead, according to aspects ofthis disclosure, the first vehicle model 126 may be based at least inpart on a model in which the point of zero velocity is centeredlaterally, as in conventional two-wheel steering models, but is alsocentered longitudinally between a first axis extending through theleading wheels 110 and a second axis extending through the trailingwheels 112.

Assuming a wheelbase of the vehicle 102 is the distance between thefirst axis and the second axis, just described, the first vehicle model126 assumes a two-wheel steering model for a vehicle having half theactual wheelbase of the vehicle 102. Accordingly, conventional planningprogramming and systems, e.g., for two-wheel (front-wheel) steeringvehicles, can be implemented for the four-wheel steering of the vehicle102 by modeling the vehicle 102 as having half its actual wheelbase.Implicitly, this model assumes that the vehicle 102 has mirroredsteering about a lateral axis at the half-wheelbase. That is, in thefirst vehicle model 126, if the leading wheels are turned 10-degrees ina first direction, the trailing wheels would be turned 10-degrees in theopposite direction.

The trajectories 124 are generated by the planning component 118 basedon the first vehicle model 126. For instance, the planning component 118may receive a current state of the vehicle, e.g., including a steeringangle of the leading wheels 110. The planning component 118, using thefirst vehicle model 126, can determine an updated leading wheel steeringangle and an acceleration for the vehicle 102 to perform as desired,e.g., to attain a desired destination or state. Moreover, by integratingthe steering and acceleration commands forward, additional vehiclestates can be determined for future times. Thus, the trajectories 124can include information about one or more of these vehicle states, e.g.,position(s) of the vehicle 102, yaw rate(s) associated with the vehicle102 at the position(s), velocity(ies) at the position(s), or the like,as well as the steering commands and acceleration commands associatedwith the positions. In examples of this disclosure, the steeringcommands may include only steering commands for the leading wheels, onlysteering commands for the trailing wheels, or steering commands for boththe leading wheels and the trailing wheels. In examples, however, onlythe leading steering angles may be included in the trajectories 124. Asnoted above, in implementations of the first vehicle model 126, the rearsteering commands are the opposite, e.g., the additive inverse, of thefront steering commands. Accordingly, by outputting only the frontsteering angle, functionality of the planning component 118 can besimilar to that of a conventional planning system used for two-wheelsteering. Accordingly, the techniques described herein for four-wheelsteering may be integrated into existing two-wheel steeringarchitectures with minimal change to that architecture. Additionaldetails of the functionality of the planning component 118 are describedfurther below in connection with FIG. 3. Moreover, other vehicle modelsmay be used as the first vehicle model 126, including models in whichthe trailing steering angle is not necessarily the additive inverse.

The vehicle system controllers 120 can receive the trajectories 124 andperform actions to implement the trajectories 124. Specifically, thevehicle system controllers 120 may include instructions stored on amemory that, when executed, cause any number of controllers to controlaspects of the vehicle 102. FIG. 1 specifically shows a leading wheelssteering controller 128 and a trailing wheels steering controller 130.As described herein, the vehicle 102 is configured for four-wheelsteering, such that the leading wheels 110 are steerable independentlyof the trailing wheels 112, and vice versa. Accordingly, and asillustrated, the leading wheels steering controller 128 is configured togenerate one or more leading wheel commands 130, e.g., as leading wheelssteering angles δ_(L) for the leading wheels 110, and the trailingwheels steering controller 130 is configured to generate one or moretrailing wheels commands 134, e.g., as trailing wheels steering anglesδ_(T) for the trailing wheels 112.

Although FIG. 1 shows the vehicle system controllers 120 as includingonly the leading wheels steering controller 128 and the trailing wheelssteering controller 130, the vehicle system controllers 112 will includeadditional controllers for controlling additional aspects of the vehicle102. Without limitation, and as discussed above, the trajectories 124may include information about an acceleration of the vehicle 102. Anot-illustrated controller of the vehicle system controllers 120 maycontrol an acceleration of the vehicle 102. Thus, in someimplementations, the vehicle system controllers 120 may receive thetrajectories 124 generated by the planning component 118 and controlaspects of the vehicle 102 to implement the trajectories 124. As notedabove, in some implementations of this disclosure, the trajectories 124may include only leading wheels steering commands, e.g., correspondingto the leading wheels steering angle δ_(L). The first vehicle model 126implicitly assumes the trailing wheels steering angle δ_(T) to be theadditive inverse of the leading wheels steering angle δ_(L), so in someexamples, the trailing wheels steering controller 130 may generatetrailing wheel commands 134 to cause the trailing wheels 112 to mirror,e.g. about a lateral axis of the vehicle 102, the steering instructed bythe leading wheel commands 132. However, in some examples it may bedesirable to have the trailing wheels 112 steered other than oppositethe leading wheels 110. In the illustration of FIG. 1, the trailingwheels 112 are steered at approximately the same angle as the leadingwheels 110.

In some instances, the vehicle system controllers 120, and morespecifically, the leading wheels steering controller 128 and/or thetrailing wheels steering controller 130, may be configured to determinethe leading wheels commands 132 and/or the trailing wheel commands 134from the trajectories 124. In other implementations, however, thetracking component 122 may include functionality to continuously executethe trajectories 124, e.g., by generating tracking commands, as theleading wheel commands 132 and/or the trailing wheel commands 134 inresponse to real-time or near real-time data from the vehicle 102, e.g.,received and/or generated as sensor data 136.

The tracking component 122 is generally configured to determine whetherthe vehicle 102 is properly executing the trajectories 124. For example,the tracking component 122 of the vehicle 102 may include instructionsstored on a memory that, when executed by processor(s), configure theprocessor(s) to determine how to implement the trajectories 124 andcalculate deviations from target states of the vehicle 102, e.g., thevehicle states provided in the trajectories 124. In examples, thetracking component 122 can receive the sensor data 136 and determinewhether the vehicle 102 comports with a target heading, a targetsteering angle, a target steering rate, a target position, a targetvelocity, a target acceleration, and/or other target conditions.Moreover, the tracking component 122 can determine corrective actions toaccount for detected variations from the target condition(s). Althoughthe tracking component 122 can be configured to determine and/or correctfor any target conditions, aspects of this disclosure may beparticularly directed to determining four-wheel steering commands, e.g.,the leading wheel commands 132 and the trailing wheel commands 134, tocorrect for steering-related or tracking errors, such as heading,lateral offset, or the like.

As illustrated in FIG. 1, the tracking component 122 can implement asecond vehicle model 138 to determine the leading wheel commands 132 andthe trailing wheel commands 134. More particularly, the second vehiclemodel 138 may be a dynamic vehicle model that generates leading wheelssteering angles (δ_(L)) and trailing wheels steering angles (δ_(T))based on current position, vehicle orientation, or the like, e.g., asdetermined from the sensor data 136. As detailed further herein, thesecond vehicle model 138 may determine the steering angles (δ_(L),δ_(T)) as a summation of a feedforward term and a feedback term, asconstrained by vehicle and/or safety limitations. Additional details ofthe tracking component 122 are disused further below in detail,including with reference to FIG. 4.

FIG. 2 is a schematic top view of an example vehicle 200, which maycorrespond to the example vehicle 102 shown in FIG. 1. As illustrated,the example vehicle 200 includes a first wheel 204(1), a second wheel204(2), a third wheel 204(3), and a fourth wheel 204(4) (collectively,herein, “the wheels 204”) arranged proximate corners of the vehicle 200.For instance, a frame 206 of the vehicle 200 may define a generallyrectangular shape, with the wheels 204 arranged proximate cornersthereof. More specifically, the first wheel 204(1) and the second wheel204(2) are relatively closer to a first end 208 of the vehicle and thethird wheel 204(3) and the fourth wheel 204(4) are relatively closer toan opposite, second end 210 of the vehicle 200. In the example, thefirst wheel 204(1) and the second wheel 204(2) are spaced a firstdistance from a lateral axis 212 of the vehicle 200 and the third wheel204(3) and the fourth wheel 204(4) are spaced an equal distance from thelateral axis 212. Moreover, the first wheel 204(1) and the second wheel204(2) are equidistant from a longitudinal axis 214 of the vehicle 200.Similarly, the third wheel 204(3) and the fourth wheel 204(4) areequidistant from the longitudinal axis 214.

As also illustrated in FIG. 2, the vehicle 200 includes a first steeringcontroller 216 associated with the first wheel 204(1) and the secondwheel 204(2) and a second steering controller 218 associated with thethird wheel 204(3) in the fourth wheel 204(4). In examples, the firststeering controller 216 may be one of the leading wheels steeringcontroller 128 or the trailing wheels steering controller 130 discussedabove with reference to FIG. 1. The second steering controller 218 maybe the other of the leading wheels steering controller 128 and thetrailing wheels steering controller 130.

Although the first steering controller 216 is illustrated as beingassociated with both the first wheel 204(1) and the second wheel 204(2),and the second steering controller 218 is illustrated as beingassociated with both the third wheel 204(3) and the fourth wheel 204(4),this is for example only. In other examples, each of the wheels 204 mayhave its own steering controller. Although implementations of thisdisclosure include implementing a first steering angle for leadingwheels of a vehicle, such as the vehicle 200, and a second steeringcommand for trailing wheels of the vehicle, such controls may beimplemented through individual steering controllers at the wheels.Moreover, by controlling each of the wheels 204 independently of allother wheels, the vehicle 200 may be provided with greatermaneuverability. Without limitation, while the vehicle 200 is intendedto generally move forward in either direction along the longitudinalaxis 214, in other instances, the vehicle may move generally forward ineither direction along the lateral axis 212. Such maneuverability may befacilitated by executing a first steering angle at the second wheel204(2) and the fourth wheel 204(4) and a second steering angle at thefirst wheel 204(1) and the third wheel 204(3).

The vehicle 200 also includes a vehicle computing system 220, which maybe the same as or similar to the vehicle computing system 116 discussedabove in connection with FIG. 1. As schematically illustrated, thevehicle computing system 220 can include a planning component 222 and atracking component 224. The planning component 222 may include the samefunctionality as the planning component 118, and the tracking component224 may include the same functionality as the tracking component 122,discussed above. The vehicle computing system 220 may be incommunication with the first steering controller 216 and the secondsteering controller 218, e.g., to provide control commands and/or toreceive information from the respective steering controllers.

As detailed herein, the planning component 222 may be configured todetermine trajectories, like the trajectories 124, using a first modelof the vehicle 200. In some examples, the first model is a kinematicvehicle model and is used to determine a series of vehicle states forthe vehicle 200. These states are passed to the tracking component 224,which, along with the first steering controller 216 and the secondsteering controller 218, implement the trajectories. In examples, thetracking component 224 can determine a first steering angle, e.g., to beimplemented by the first steering controller 216 at the first wheel204(1) and the second wheel 204(2), and a second steering angle, e.g.,to be implemented by the second steering controller 216 at the thirdwheel 204(3) and the fourth wheel 204(4). The tracking component 224 mayimplement a second vehicle model, e.g., a dynamic vehicle model, todetermine the first and second steering angles. In examples, the secondmodel can be applied iteratively to continuously update the steeringangles, e.g., in real-time or near real-time.

FIG. 3 represents an example process 300 for generating a trajectory foran autonomous vehicle. In particular, FIG. 3 includes textual andgraphical flowcharts illustrative of the process 300, according toimplementations of this disclosure. In some examples, the process 300can be implemented using components and systems illustrated in FIG. 1and/or FIG. 2 and described above, such as the planning component 118and/or the planning component 222, although the process 200 is notlimited to being performed by such components. Moreover, the componentsand systems of FIGS. 1 and 2 are not limited to performing the process300.

In more detail, the process 300 can include, at an operation 302,receiving a measured leading steering angle and a destination for thevehicle. For example, sensors at the wheels of a vehicle can generate ameasurement of the steering angle of the leading wheels and the vehiclecan include one or more computing systems that generate a planned pathalong which the vehicle is to travel. In an example accompanying theoperation 302, a vehicle 304 is an autonomous vehicle, which may thevehicle 102 and/or the vehicle 200 described above. As illustrated inthe example, the vehicle 304 is generally travelling in the directionindicated by arrow 306. Accordingly, the vehicle 304 includes leadingwheels 308 proximate a first (leading) end 310 of the vehicle 304 andtrailing wheels 312 proximate a second (trailing) end 314 of the vehicle304. The example accompanying the operation 302 also illustrates adestination 316 and an example path along which the vehicle 304 is totravel to reach the destination 316. In examples, the planned path maybe determined using information from a localization system, a perceptionsystem, a prediction system, and/or other aspects of a planning systemin place on the vehicle 304 or accessible by the vehicle 304. Theplanned path may be a desired route along which the vehicle 304 is totravel in an environment, such as the environment 100 discussed above.In this example, the planned path is illustrated as a continuous linehaving varied curvature. In operation, the vehicle 304 may be controlledto maintain a centered position on the planned path. However, theplanned path is not limited to the path illustrated. In some examples,the planned path may be an envelope or other shape having a width, e.g.,laterally relative to the illustrated path and which the lateral extentsof the vehicle 304 are to maintain during operation of the vehicle 304.As used herein, a planned path is generally understood to be a segmentor area relative to which the vehicle 304 is to travel. For instance,the planned path may be determined to avoid obstacles or objects in anenvironment of the vehicle 304, including but not limited to othervehicles, pedestrians, or the like, to maintain the vehicle 304 and alane or other segment defined by a road or drivable surface, or tootherwise guide the vehicle 304.

As also illustrated in the example accompanying the operation 302, theleading wheels 308 are angled relative to the direction indicated by thearrow 306. More specifically, the leading wheels 308 are angled relativeto a longitudinal axis of the vehicle by a steering angle, δ_(M). Thesteering angle may be measured using conventional sensors or other meansassociated with the leading wheels 308, a steering system associatedwith the leading wheels 308, or otherwise. Accordingly, the steeringangle δ_(M) is the actual, current angle at which the leading wheels 308are steered. In the example accompanying the operation 302, the trailingwheels 312 are also steered relative to the longitudinal axis of thevehicle 304. In this instance, the trailing wheels 312 are steered at adifferent, e.g., smaller angle than the steering angle δ_(M) measured atthe leading wheels 310. However, the steering angle associated with thetrailing wheels 312 may not be used in the process 300.

At an operation 318, the process 300 includes determining a model of thevehicle. For example, because the vehicle 304 includes independentsteering for the leading wheels 308 and the trailing wheels 312, thevehicle model for determining trajectories for the vehicle 304 isdifferent from a model used to determine trajectories of a vehicleconfigured for front-wheel steering only. An example accompanying theoperation 318 illustrates aspects of a representation 320 of the vehicle304, and the representation 320 is illustrative of some aspects of amathematical model used to generate trajectories, as described herein.The mathematical model may be the first vehicle model 126 discussedabove. The mathematical model may be a kinematic model of the vehicle304.

The representation 320 conceptualizes aspects of the model. Morespecifically, the representation a distance between the first axis 322and the second axis 324 is a wheelbase of the vehicle 304, representedby x in the example. As illustrated, the representation 320 alsoincludes a point 326 that is equally spaced, e.g., in the longitudinaldirection, between the first axis 322 and the second axis 324. The point326 is also disposed along a longitudinal axis 328 of the vehicle 304.As also shown in the example accompanying the operation 318, therepresentation 320 shows a steering angle δ associated with the leadingwheels 310. Moreover, the trailing wheels are indicated as beingdisposed at the steering angle −δ, that is, the negative or opposite ofthe steering angle δ of the leading wheels. In implementations of thisdisclosure, the mathematical model used to determine trajectories usesonly the leading steering angle (as received at the operation 302) thefront or leading end of the vehicle and determines only a front steeringangle for inclusion in the trajectories, as described herein. The modelassumes that the point 326 is a point at which the vehicle 304 has zerolateral velocity. Accordingly, the model implicitly includes mirroredsteering, as shown in the representation 320. That is, the trailingwheels are mirrored relative to a lateral axis of the vehicle 304.

At an operation 330, the process 300 includes planning a trajectoryusing the model. For example, a planning component implements themathematical model to determine one or more future vehicle states forfollowing the planned path to the destination 316. In the example ofFIG. 3, a planning component 332 receives the measured leading steeringangle, δ_(M), and the destination 316, and executes a model 334 todetermine a trajectory 336. The planning component 332 may also receiveadditional information about the vehicle 304 to execute the model 334 todetermine the trajectory 336. Without limitation, the planning componentcan receive information about a current pose of the vehicle, e.g., acurrent position of the vehicle 304, a current velocity of the vehicle304, a current acceleration of the vehicle 304, a current yaw rate ofthe vehicle 304, in addition to the current leading steering angle,δ_(M), and determine, using the kinematic model 334, the trajectory 336as one or more leading wheel steering angles, δ_(L), one or moreaccelerations for the vehicle, and one or more vehicle states. Forinstance, the planning component 332, using the model 334, can determinea leading wheel steering angle, δ_(L), and an acceleration for thevehicle 304, e.g., as vehicle commands, and integrate those commandsforward using the model 334 over some predetermined time period todetermine a plurality of vehicle states (e.g., positions (for instance,of the point 326), velocities, yaw rates, and/or other characteristics)as the trajectory. For example, the trajectory may include a pluralityof steering commands, acceleration commands, and vehicle statesdetermined at a frequency.

The model 334, as represented by the representation 320, may be readilyintegrated into planning architectures that also are configured for usein vehicles configured for two-wheel steering. For instance, because themodel 334 is a kinematic model based on a point of zero lateralvelocity, e.g., the point 326, the model 334 may be used for othervehicle types at which a point of zero lateral velocity can beidentified. In some examples, trajectories can be determined for avehicle configured only for two-wheel, e.g., the leading wheels,steering by identifying the point of zero lateral velocity at a pointcentrally-located on the rear wheel axis. In this example, the trailingsteering angle will always be zero, so a kinematic model could beimplemented with the point of zero velocity between the trailing wheels.Although not illustrated in FIG. 3, the trajectory 336 may be forwardedto a vehicle controller, for implementation.

FIG. 4 represents an example process 400 for executing a trajectory atan autonomous vehicle using four-wheel steering. In particular, FIG. 4includes textual and graphical flowcharts illustrative of the process400, according to implementations of this disclosure. In some examples,the process 400 can be implemented using components and systemsillustrated in FIG. 1 and/or FIG. 2 and described above, such as thetracking component 122 and/or the tracking component 224, although theprocess 400 is not limited to being performed by such components.Moreover, the components and systems of FIGS. 1 and 2 are not limited toperforming the process 400.

At an operation 402, the process 400 includes receiving a trajectory andcurrent state information. A graphical example accompanying theoperation 402 includes a graphical representation of a trajectory 404and of vehicle data 406. In the illustration, the trajectory 404includes a leading wheels steering angle, δ_(L), an acceleration, a, andone or more vehicle states, although the trajectory 404 may include anyadditional or different information. The trajectory may be thetrajectory 336 generated by the process 300 discussed above, althoughthe process 400 is not limited to using the trajectory 336. The vehicledata may include current information about the vehicle, includinginformation of a current pose of the vehicle, such as the currentvelocity, yaw, position, yaw rate, or the like. The state informationmay be defined at a center of gravity of the vehicle.

At an operation 408, the process 400 includes determining first leadingsteering angles and first trailing steering angles. As described, thisdisclosure generally relates to four-wheel steering, in which twoleading wheels (such as the leading wheels 110) are steeredindependently of two trailing wheels (such as the trailing wheels 112).Thus, the leading wheels are steered according to a leading wheelsteering angle, δ_(l), and the trailing wheels are steered according toa trailing wheel steering angle, δ_(t). The operation 408 may beassociated with determining a feedforward steering command, whereas theoperation 422, discussed in more detail below, may be associated withdetermining a feedback command.

An example accompanying the operation 408 illustrates a firstrepresentation 410 and a second representation 412 of a vehicle 414 todemonstrate steering angle ratios for determining first leading steeringangle δ_(l,1), and a first trailing steering angle δ_(t,1). Morespecifically, in both the first representation 410 and the secondrepresentation 412, leading wheels 416 of the vehicle 414 are steered ata leading steering angle δ_(l), and trailing wheels 418 of the vehicle414 are steered at a trailing steering angle δ_(t). In the firstrepresentation 410, the trailing wheels 418 mirror the leading wheels416 about a longitudinal axis of the vehicle 414. Accordingly, thetrailing steering angle δ_(t) is the additive inverse of the leadingsteering angle δ_(l) or, stated differently, δ_(t)=−δ_(l) in the firstrepresentation 410 and a ratio of the leading trailing steering angle tothe leading steering angle is −1. In the second representation 412, thetrailing steering angle δ_(l) and the leading steering angle δ_(t) areequal. Accordingly, the ratio of the leading steering angle to thetrailing steering angle is 1. The first representation 410 and thesecond representation 412 illustrate only two states of the vehicle 414;the trailing wheels 418 may be steered at any ratio to the leadingwheels 416, e.g., any ratio between −1 and 1.

In aspects of this disclosure, the operation 408 includes determining afirst leading steering angle δ_(l,1), and a first trailing steeringangle δ_(t,1) based at least in part on the ratio of the trailingsteering angle to the leading steering angle. For instance, atrelatively lower speeds, a negative ratio of δ_(t)/δ_(l) may provideincreased maneuverability, e.g., as a higher turning radius, withoutsacrificing stability. On the contrary, at relatively higher speeds, apositive ratio of δ_(t)/δ_(l) may provide increased stability insteering, e.g., during lane changes and/or cornering. A graphicalrepresentation 420 demonstrates an example plot of the ratio ofδ_(t)/δ_(l) that may be used, at least in part, to determine the firstleading steering angle δ_(l,1), and the first trailing steering angleδ_(t,1).

In examples of this disclosure, the operation 408 may determine thefirst leading steering angle δ_(l,1), and the first trailing steeringangle δ_(t,1) based at least in part on a dynamic model of the vehicle.In one example, the dynamic model may be based at least in part on asteady state single track, e.g., a bicycle model, with a nonlinear Fialatire model. A lateral bicycle model has two vehicle states—yaw rate

and sideslip β. Because the leading wheels and the trailing wheels areindependently steerable, the dynamic model has two unknowns and the twovehicle states. The first leading steering angle δ_(l,1), and the firsttrailing steering angle δ_(t,1) can be determined for desired vehiclestates and using tire slip equations, e.g., according to the followingequations (1) and (2):

δ_(l,1) =ak−α _(l)  (1)

δ_(t,1) =bk−α _(t)  (2)

In Equations (1) and (2), a is the distance from the center of mass ofthe vehicle to a first axis extending through the leading wheelsrotational axis (e.g., a distance along a longitudinal direction), b isthe distance from the center of mass of the vehicle to a second axisextending through the trailing wheels rotational axis, k is a curvatureof a path to be followed, e.g., the planned path 114 or the planned path316, and α is a slip angle. More specifically, the slip angles may bederived from a leading and trailing tire lateral force assuming a staticweight distribution and steady state force balance, e.g., based onlateral acceleration, lateral force at the leading and trailing wheelsand a mass of the vehicle. The forces may be obtained from a tire curvegiven by a nonlinear tire model to determine the tire slip angles. Insome implementations, the tire model may be a Fiala tire model, whichrequires only two parameters—cornering stiffness C and friction μ, bothof which are measured from vehicle data, such as the vehicle data 406.

At an operation 422, the process 400 includes determining second leadingsteering angles and second trailing steering angles. For example, theoperation 408 may determine expected steering angles of a vehicle modelto track a desired path, the operation 422 may compare a current stateof the vehicle to the path, e.g., where the vehicle should be along thepath, and determine corrective steering commands. These correctivesteering commands include a second leading steering angle δ_(l,2), and asecond trailing steering angle δ_(t,2). An example accompanying theoperation 422 illustrates a current state of the vehicle 414 relative toa planned path 424. The planned path 424 may be the planned path 114and/or the planned path 316. As also illustrated in the example, thevehicle 414 has a center of mass 426 and is travelling in a longitudinaldirection illustrated by the arrow 428. A distance between the center ofmass 426 and the planned path 424 is a lateral offset, e, designated byan arrow 430. The arrow 430 is perpendicular to the planned path 424.The example accompanying the operation 422 also illustrates an offsetangle Δψ, which is an angular offset between the planned path 424 andthe actual heading of the vehicle 414. Stated differently, e representsa lateral error and Δψ represents a heading error of the vehicle 414,which together represent a tracking error.

The operation 422 includes using the lateral error and the heading errorto determine the second leading steering angle δ_(l,2), and the secondtrailing steering angle δ_(t,2). More specifically, the operation 422can include solving Equations (3) and (4):

δ_(l,2) =−k ₁ e−k ₂Δψ  (3)

δ_(t,2) =−k ₃ e−k ₄Δψ  (4)

In Equations (3) and (4), the terms k₁, k₂, k₃, and k₄ are proportionalgains.

At an operation 432, the process 400 includes determining steeringcommands based at least in part on the first and second leading andtrailing steering angles. As described herein, the operation 408determines the first leading steering angle δ_(l,1), and the firsttrailing steering angle δ_(t,1) based on a dynamic model of the vehicle,e.g., as estimated or feedforward commands, and the operation 422determines the second leading steering angle δ_(l,2), and the secondtrailing steering angle δ_(t,2) based on measured errors, e.g., ascorrective or feedback commands. In examples, the operation 432 candetermine a leading wheels steering angle, δ_(l), and a trailing wheelssteering angle δ_(t), as a sum including the first and second leadingwheels steering angles and the first and second trailing wheels steeringangles, respectively. In examples, the leading wheels steering angle,δ_(l), and the trailing wheels steering angle, δ_(t), are determinedusing Equations (5) and (6):

δ_(l)=δ_(l,1)+δ_(l,2)+δ_(l,LH)  (5)

δ_(t)=δ_(t,1)+δ_(t,2)+δ_(t,LH)  (6)

Thus, according to Equations (5) and (6), the commanded leading wheelssteering angle is a sum of the leading wheels steering angle determinedfrom a dynamic model of the vehicle 414, e.g., at the operation 408, andthe leading wheels steering angle determined from state errors of thevehicle 414, e.g. at the operation 422. Similarly, the trailing wheelssteering angle is a sum of the trailing wheels steering angle determinedfrom the dynamic model and the trailing wheels steering angle determinedfrom the state errors. Equations (5) and (6) also include a third term,i.e., δ_(l,LH) and δ_(t,LH). These third terms are limit handling termsand account for situations in which the determined leading wheelssteering angle and/or the trailing wheels steering angle calculated fromthe first steering angles and the second steering angles cannot beimplemented at the vehicle. For instance, steering of the leading wheelsand/or the trailing wheels can be limited to some maximum steering angleand the limiting handling terms can include modification angles thatmodify the calculated leading and/or trailing steering angles, asdetailed herein. The maximum steering angle may be based on the physicalcapabilities of the vehicle and/or based on a predetermined or desiredconstraint applied to the vehicle, e.g., regardless of the actualcapabilities of the vehicle. For instance, the maximum steering anglesmay be determined based on preferred performance characteristics,passenger comfort, or other parameters.

FIG. 5 represents an example process 500 for executing a trajectory atan autonomous vehicle using four-wheel steering. In particular, FIG. 5includes textual and graphical flowcharts illustrative of the process500, according to implementations of this disclosure. The process 500may be performed along with the process 400, e.g., the processes 400,500 may be component processes of an overall process for executing atrajectory. In some examples, the process 500 can be implemented usingcomponents and systems illustrated in FIG. 1 and/or FIG. 2 and describedabove, such as the tracking component 122 and/or the tracking component224, although the process 500 is not limited to being performed by suchcomponents. Moreover, the components and systems of FIGS. 1 and 2 arenot limited to performing the process 500.

At an operation 502, the process 500 includes determining a leadingsteering angle and a trailing steering angle. For instance, theoperation 502 can include performing the process 400 to estimate frontand rear steering angles based on a dynamic model of a vehicle andaltering those estimated angles using corrective steering angles basedon errors calculated from a predicted path. An example accompanying theoperation 502 graphically depicts steering angle data 504 comprising aleading wheel steering angle, δ_(l), and a trailing wheel steering angleδ_(t).

At an operation 506, the process 500 includes determining that theleading and/or the trailing steering angles exceed maximum steeringlimits. An example accompanying the operation 506 includes a graphicalrepresentation 508 illustrating a vehicle 510 having leading wheels 512and trailing wheels 514. In the representation, the leading wheels 512are steerable relative to a longitudinal axis 516 up to a leading wheelmaximum steering angle, δ_(l,max), in either direction, e.g., over arange from δ_(l,max) to −δ_(l,max) (shown in dotted lines). Similarly,the trailing wheels 514 are steerable relative to a longitudinal axis516 up to a trailing wheel maximum steering angle, δ_(t,max), in eitherdirection, e.g., over a range from δ_(t,max) (shown in dashed lines) to−δ_(t,max). In some examples, the leading wheel maximum steering anglemay be different from the trailing wheel maximum steering angle. Forinstance, the trailing wheel maximum steering angle may be smaller thanthe leading wheel maximum steering angle. The operation 506 may includedetermining that one or both of the steering angles δ_(l), δ_(t)included in the steering angles data 504 (e.g., determined using theprocess 400) are outside the maximum steering angles, δ_(l,max),δ_(t,max), respectively.

At an operation 518, the process 500 includes determining a leadingsteering correction angle and/or a trailing steering correction angle.For example, the dynamic model used to determine the estimated steeringangles, e.g., at the operation 408, can also be used to determine limithandling correction terms δ_(l,LH), δ_(t,LH), that can be summed withthe estimated steering angles and the corrective steering angles whenone or both of the steering angles are beyond the maximum limits. Inmore detail, the Equations (1) and (2) used to determine the estimatedor feedforward angles are simplified in that they are based on theassumption that the vehicle has a steady-state heading error of zero,e.g., sideslip, β, is zero, such that the leading and trailing wheelsare chosen arbitrarily. The Equations (1) and (2) can be rewritten toinclude the sideslip, as equations (7) and (8):

δ_(l,1) =ak−α _(l)+β_(ffw)+δ_(l) ^(i)  (7)

δ_(t,1) =bk−α _(t)+β_(ffw)+δ_(t) ^(i)  (8)

In Equations (7) and (8), β_(ffw) is the sideslip of the vehicle and theδ terms are constant, unknown offsets.

During nominal operation, e.g., as in the process 400, when the leadingand trailing wheel angles are within the steering limits of the vehicle,steady-state sideslip is set to zero, such that the vehicle will have asteady-state heading error of zero and the leading and trailing wheelangles are arbitrarily chosen. However, when one of the angles hits alimit, the process can no longer arbitrarily determine the leading andtrailing steering angles. Because the tire slip terms, α, are determinedusing lateral force equations, as introduced above, the required forceat the saturated wheels can still be met in the saturated axle usingvehicle sideslip. More specifically, the process 500 can allow anon-zero steady-state sideslip. Conceptually, and as illustrated in anexample accompanying the operation 518, the longitudinal axis 516 of thevehicle 510 is angled relative to a planned path by the non-zerosideslip, β. That is, the vehicle 510 is no longer pointing tangent tothe planned path 520 as in Equations 1 and 2, in which the sideslip wasassumed to be zero.

In more detail, define a saturation function, sat(x,lb,ub), which boundsa value x between a lower bound, lb, and an upper bound, ub. Thesaturated steering angles can then be given by the Equations (9) and(10):

δ_(l,1) =sat(δ_(l,nom),−δ_(l,max),δ_(l,max))  (9)

δ_(t,1) =sat(δ_(t,nom),−δ_(t,max),δ_(t,max))  (10)

For instance, the nominal leading steering angle δ_(l,nom) and thenominal trailing steering angle δ_(t,nom) may be the steering anglesδ_(l), δ_(t) determined by the process 400. The steady-state sideslip iscalculated using Equation (11) during leading wheel saturation:

β=δ_(l,sat)−δ_(l,nom)  (11)

And is calculated using Equation (12) during trailing wheel saturation:

β=δ_(t,sat)−δ_(t,nom)  (12)

Substituting these sideslip Equations (11) and (12) into Equations (9)and (10), respectively, yields equations (13) and (14):

δ_(l,1)=sat[(δ_(l,nom)−(δ_(t,nom)−δ_(t,sat)),−δ_(l,max),δ_(l,max)]  (13)

δ_(t,1)=sat[(δ_(t,nom)−(δ_(l,nom)−δ_(l,sat)),−δ_(t,max),δ_(t,max)]  (14)

In operation of the vehicle 510 implementing these steering angles, ifboth the front and the rear steer angels are saturated, the steady-statesideslip is not updated (or is set to the previously calculated value),effectively capping the calculated steady-state sideslip at the valuethat occurs when both steer angles saturate.

At an operation 522, the process 500 includes determining steeringcommands based at least in part on the steering corrections. Asillustrated in the example accompanying the operation 518, the operation518 can include determining limit handling steering angles that act tomodify vehicle behavior during steering saturation. For instance, theseangles may be modification angles that modify the feedback or correctiveangles, e.g., determined at the operation 518 to maintain vehiclestability during steering saturation. The limit handling angles may begiven by Equations (15) and (16), as follows:

δ_(l,LH) =−k _(l,sat)[(δ_(t,1)+δ_(t,2)−sat(δ_(t,1)+δ_(t,2)),−δ_(t,max),δ_(t,max)]  (15)

δ_(t,LH) =−k _(t,sat)[(δ_(l,1)+δ_(l,2)−sat(δ_(l,1)+δ_(l,2)),−δ_(l,max),δ_(l,max)]  (16)

In Equations (15) and (16), k_(l,sat) and k_(t,sat) are saturation gainschosen to result in stable closed-loop dynamics and desirable systemperformance.

The process 500 may also include integral controlling, which allows tovehicle to reject constant steering offsets as well as allow forsteady-state lateral errors during a ramp steer. When the steeringangles remain below their limits, the integral steering angles integrateboth lateral error and the sum of heading error and steady-statesideslip. During a saturation event, the steering angels integrate onlylateral error. This is consistent with functionality of someconventional two-wheel steering integral controllers. The vehiclecomputing system can include logic to switch between integral angles.For instance, if the there is no front saturation and no rearsaturation:

δ_(l) ^(i)=∫[(−k _(ei,l) e−k _(ψi,l)(Δψ+β_(ffw))]dt  (17)

δ_(t) ^(i)=∫[(−k _(ei,t) e−k _(ψi,t)(Δψ+β_(ffw))]dt  (18)

If there is rear saturation, but not front saturation:

δ_(l) ^(i)=ƒ[−k _(f,i) ^(sat) e]dt  (19)

And if there is front saturation, but not rear saturation:

δ_(t) ^(i)=ƒ[−k _(t,i) ^(sat) e]dt  (20)

As will be appreciated, the integral steering angles will be included inthe feedforward angles, e.g., the angles δ_(l,1), δ_(t,1).

FIG. 6 depicts a block diagram of an example system 600 for implementingthe techniques described herein. In at least one example, the system 600can include a vehicle 602, which may be the same as or different fromthe vehicle 102 shown in FIG. 1, the vehicle 200 shown in FIG. 2, thevehicle 304 shown in FIG. 3, the vehicle 414 shown in FIG. 4, and/or thevehicle 510 shown in FIG. 5. The vehicle 602 can include one or morevehicle computing devices 604, one or more sensor systems 606, one ormore emitters 608, one or more communication connections 610, one ormore drive modules 612, and at least one direct connection 614.

The vehicle computing device(s) 604 also include one or more processors616 and memory 618 communicatively coupled to the one or more processors616. In the illustrated example, the vehicle 602 is an autonomousvehicle. However, the vehicle 602 could be any other type of vehicle, orany other system having at least one sensor (e.g., a camera enabledsmartphone). In the illustrated example, the memory 618 of the vehiclecomputing device 604 stores a localization component 620, a perceptioncomponent 622, a planning component 626, a tracking component 626,system controllers 628, and steering data 630. Moreover, the systemcontrollers 628 include first steering controller 632 and a secondsteering controller 634. Though depicted in FIG. 6 as residing in thememory 618 for illustrative purposes, it is contemplated that thelocalization component 620, the perception component 622 (and/orcomponents thereof), the planning component 626, the tracking component626, the system controllers 628, and the steering data 630 canadditionally, or alternatively, be accessible to the vehicle 602 (e.g.,stored on, or otherwise accessible by, memory remote from the vehicle602).

In at least one example, the localization component 620 can includefunctionality to receive data from the sensor system(s) 606 to determinea position and/or orientation of the vehicle 602 (e.g., one or more ofan x-, y-, z-position, roll, pitch, or yaw). For example, thelocalization component 620 can include and/or request/receive a map ofan environment and can continuously determine a location and/ororientation of the autonomous vehicle within the map. In some instances,the localization component 620 can utilize SLAM (simultaneouslocalization and mapping), CLAMS (calibration, localization and mapping,simultaneously), relative SLAM, bundle adjustment, non-linear leastsquares optimization, or the like to receive image data, lidar data,radar data, IMU data, GPS data, wheel encoder data, and the like toaccurately determine a location of the autonomous vehicle. In someinstances, the localization component 620 can provide data to variouscomponents of the vehicle 602, such as the planning component 626, todetermine a current position of the autonomous vehicle 602 forgenerating a trajectory.

In some instances, the perception component 622 generally includesfunctionality to perform object detection, segmentation, and/orclassification. In some examples, the perception component 622 canprovide processed sensor data that indicates a presence of an objectthat is proximate to the vehicle 602 and/or a classification of theobject as an object type (e.g., car, pedestrian, cyclist, animal,building, tree, road surface, curb, sidewalk, unknown, etc.). Inadditional and/or alternative examples, the perception component 622 canprovide processed sensor data that indicates one or more characteristicsassociated with a detected object (e.g., a tracked object) and/or theenvironment in which the object is positioned. In some examples,characteristics associated with the object can include, but are notlimited to, an x-position (global and/or local position), a y-position(global and/or local position), a z-position (global and/or localposition), an orientation (e.g., a roll, pitch, yaw), an object type(e.g., a classification), a velocity of the object, an acceleration ofthe object, an extent of the object (size), etc. Characteristicsassociated with the environment can include, but are not limited to, apresence of another object in the environment, a state of another objectin the environment, a time of day, a day of a week, a season, a weathercondition, an indication of darkness/light, etc.

In general, the planning component 626 can determine a path for thevehicle 602 to follow to traverse through an environment. The planningcomponent 626 can be, or include functionality ascribed to, the planningcomponent 118 and/or the planning component 222. For example, theplanning component 626 can determine various routes and trajectories andvarious levels of detail using a vehicle model, such as the firstvehicle model 126. The planning component 626 can determine a route totravel from a first location (e.g., a current location) to a secondlocation (e.g., a target location). For the purpose of this discussion,a route can be a sequence of waypoints for travelling between twolocations. As non-limiting examples, waypoints include streets,intersections, global positioning system (GPS) coordinates, etc.Further, the planning component 626 can generate an instruction forguiding the autonomous vehicle along at least a portion of the routefrom the first location to the second location. In at least one example,the planning component 626 can determine how to guide the autonomousvehicle from a first waypoint in the sequence of waypoints to a secondwaypoint in the sequence of waypoints. In some examples, the instructioncan be a trajectory, or a portion of a trajectory. The trajectory caninclude a series of vehicle states corresponding to the waypoints andcommands for achieving those states. Such commands can include steeringangles and accelerations, for example. Without limitation, the planningcomponent 626 may receive a current steering angle, e.g., of leadingwheels of a vehicle, and determine trajectories based at least in parton the steering angle using operations discussed above in associationwith FIG. 3. The model may be a kinematic model of the vehicle, in whichthe vehicle 602 is assumed to have a point of zero velocity midwaybetween a leading axis and a trailing axis, as described herein. In someimplementations, multiple trajectories can be substantiallysimultaneously generated (e.g., within technical tolerances) inaccordance with a receding horizon technique, with one of the multipletrajectories being selected for the vehicle 602 to navigate.

The tracking component 626 includes functionality to adjust steeringcommands to follow a planned path, e.g., in accordance with a trajectorygenerated by the planning component 624. Without limitation, thetracking component 626 can be, or implement functionality ascribed to,the tracking component 122 and/or the tracking component 224. Inexamples, the tracking component 626 can use a vehicle model, such as adynamic vehicle model, to generate leading wheel steering angles andtrailing wheel steering angles for controlling the vehicle 602 to followa path. In some instances, the tracking component 224 can implement theprocesses 400, 500 to determine the steering angles. Although thetracking component 626 is illustrated as being a separate from thesystem controllers 628, in some examples the tracking component 626 canbe integrated into the system controllers 628.

The system controllers 628 can be configured to control steering,propulsion, braking, safety, emitters, communication, and other systemsof the vehicle 602, e.g., based on controls generated by the planningcomponent 624 and/or the tracking component 626. As illustrated, thesystem controllers 628 include a first steering controller 632 and asecond steering controller 634. For instance, the first steeringcontroller 632 can be used to control a steering angle of leading wheelsof the vehicle 602. The second steering controller 634 can be used tocontrol a steering angle of the trailing wheels of the vehicle, e.g.,independently of the leading wheels. The system controllers 628 cancommunicate with and/or control corresponding systems of the drivemodule(s) 612 and/or other components of the vehicle 602.

The steering data 630 can be used by the vehicle 602 in association withfour-wheel steering techniques, as described herein. For instance, thesteering data 630 can include information about steering constraints,such as maximum steering angles and/or maximum steering rate limits, forthe leading wheels and the trailing wheels. Also in implementations, thesteering data 620 can include a steering command library accessible byvarious components of the vehicle 602 to provide steering information indifferent formats, adapted for those components. Without limitation, thesteering data 620 can include data to select a steering angle and/orrate limits and to convert curvature and curvature rate information tofront and rear steering limits in radians and radians/second,respectively, for different components. For instance, the steering data630 can include conversion factors to facilitate such conversions.Moreover, in some instances, the steering data 620 can includeinformation to allow for both two-wheel and four-wheel steering, e.g.,for translation or conversion of information when switching fromtwo-wheel to four-wheel steering, or vice versa. The steering data 630may be a steering library accessible by the various components of thevehicle 602, including the localization component 620, the perceptioncomponent 622, the planning component 624, the tracking component 626,and/or other components.

As can be understood, the components discussed herein (e.g., thelocalization component 620, the perception component 622 (and itscomponents), the planning component 624, the tracking component 626, thesystem controller(s) 628, and the steering data 630) are shown anddescribed as divided for illustrative purposes. However, the operationsperformed by the various components can be combined or performed in anyother component.

The sensor system(s) 606 can include LiDAR sensors, radar sensors,ultrasonic transducers, sonar sensors, location sensors (e.g., GPS,compass, etc.), inertial sensors (e.g., inertial measurement units(IMUs), accelerometers, magnetometers, gyroscopes, etc.), cameras (e.g.,RGB, IR, intensity, depth, time of flight, etc.), microphones, wheelencoders, environment sensors (e.g., temperature sensors, humiditysensors, light sensors, pressure sensors, etc.), etc. The sensorsystem(s) 606 can include multiple instances of each of these or othertypes of sensors. For instance, the lidar sensors can include individuallidar sensors located at the corners, front, back, sides, and/or top ofthe vehicle 602. As another example, the camera sensors can includemultiple cameras disposed at various locations about the exterior and/orinterior of the vehicle 602. As another example, the radar system caninclude multiple instances of the same or different radar sensorsdisposed at various locations about the vehicle 602. The sensorsystem(s) 606 can provide input to the vehicle computing device 604.Additionally or alternatively, the sensor system(s) 606 can send sensordata, via one or more networks 636, to remote computing device(s) 638 ata particular frequency, after a lapse of a predetermined period of time,in near real-time, etc.

The emitter(s) 608 can be configured to emit light and/or sound. Theemitter(s) 608 in this example can include interior audio and visualemitters to communicate with passengers of the vehicle 602. By way ofexample and not limitation, interior emitters can include speakers,lights, signs, display screens, touch screens, haptic emitters (e.g.,vibration and/or force feedback), mechanical actuators (e.g., seatbelttensioners, seat positioners, headrest positioners, etc.), and the like.The emitters 608 in this example can also include exterior emitters. Byway of example and not limitation, the exterior emitters in this exampleinclude lights to signal a direction of travel or other indicator ofvehicle action (e.g., indicator lights, signs, light arrays, etc.), andone or more audio emitters (e.g., speakers, speaker arrays, horns, etc.)to audibly communicate with pedestrians or other nearby vehicles, one ormore of which comprising acoustic beam steering technology.

The communication connection(s) 610 can enable communication between thevehicle 602 and one or more other local or remote computing device(s).For instance, the communication connection(s) 610 can facilitatecommunication with other local computing device(s) on the vehicle 602and/or the drive module(s) 612. Also, the communication connection(s)610 can allow the vehicle to communicate with other nearby computingdevice(s) (e.g., other nearby vehicles, traffic signals, etc.). Thecommunications connection(s) 610 also enable the vehicle 602 tocommunicate with a remote teleoperations computing device or otherremote services.

The communication connection(s) 610 can include physical and/or logicalinterfaces for connecting the vehicle computing device 604 to anothercomputing device or a network, such as the network(s) 638. For example,the communication connection(s) 610 can enable Wi-Fi-based communicationsuch as via frequencies defined by the IEEE 802.11 standards, shortrange wireless frequencies such as Bluetooth®, cellular communication(e.g., 2G, 3G, 6G, 6G LTE, 5G, etc.) or any suitable wired or wirelesscommunications protocol that enables the respective computing device tointerface with the other computing device(s).

In at least one example, the vehicle 602 can include the drive module(s)612. In some examples, the vehicle 602 can have a single drive module612. In at least one example, the vehicle 602 may have multiple drivemodules 612 with individual of the drive modules 612 positioned onopposite ends of the vehicle 602 (e.g., the front and the rear, etc.).In at least one example, the drive module(s) 612 can include one or moresensor systems to detect conditions of the drive module(s) 612 and/orthe surroundings of the vehicle 602. By way of example and notlimitation, the sensor system(s) associated with the drive module(s) 612can include one or more wheel encoders (e.g., rotary encoders) to senserotation of the wheels of the drive modules, inertial sensors (e.g.,inertial measurement units, accelerometers, gyroscopes, magnetometers,etc.) to measure orientation and acceleration of the drive module,cameras or other image sensors, ultrasonic sensors to acousticallydetect objects in the surroundings of the drive module, lidar sensors,radar sensors, etc. Some sensors, such as the wheel encoders can beunique to the drive module(s) 612. In some cases, the sensor system(s)on the drive module(s) 612 can overlap or supplement correspondingsystems of the vehicle 602 (e.g., the sensor system(s) 606).

The drive module(s) 612 can include many of the vehicle systems,including a high voltage battery, a motor to propel the vehicle, aninverter to convert direct current from the battery into alternatingcurrent for use by other vehicle systems, a steering system including asteering motor and steering rack (which can be electric), a brakingsystem including hydraulic or electric actuators, a suspension systemincluding hydraulic and/or pneumatic components, a stability controlsystem for distributing brake forces to mitigate loss of traction andmaintain control, an HVAC system, lighting (e.g., lighting such ashead/tail lights to illuminate an exterior surrounding of the vehicle),and one or more other systems (e.g., cooling system, safety systems,onboard charging system, other electrical components such as a DC/DCconverter, a high voltage junction, a high voltage cable, chargingsystem, charge port, etc.). Additionally, the drive module(s) 612 caninclude a drive module controller which can receive and preprocess datafrom the sensor system(s) and to control operation of the variousvehicle systems. In some examples, the drive module controller caninclude one or more processors and memory communicatively coupled withthe one or more processors. The memory can store one or more modules toperform various functionalities of the drive module(s) 612. Furthermore,the drive module(s) 612 may also include one or more communicationconnection(s) that enable communication by the respective drive modulewith one or more other local or remote computing device(s).

In at least one example, the direct connection 614 can provide aphysical interface to couple the drive module(s) 612 with the body ofthe vehicle 602. For example, the direction connection 614 can allow thetransfer of energy, fluids, air, data, etc. between the drive module(s)612 and the vehicle. In some instances, the direct connection 614 canfurther releasably secure the drive module(s) 612 to the body of thevehicle 602.

As also illustrated, the vehicle 602 may be in communication, via thenetwork(s) 636 with the computing device(s) 638. For instance, thecomputing device(s) 638 may implement functionality described herein. Inmore detail, the computing device(s) 638 can include processor(s) 640and memory 642 storing steering data 644.

The steering data 644 may include the steering data 630. For instance,the steering data 630 may be a subset of the steering data 644, whichmay be loaded to the vehicle 602 prior to operation of the vehicle.Without limitation, the steering data 644 may include a centralizeddatabase of steering data for a fleet of vehicles, including the vehicle602, and the steering data 630 may be uploaded to the vehicle 602 asneeded.

The processor(s) 616 of the vehicle 602 and the processor(s) 640 of thecomputing device(s) 638 can be any suitable processor capable ofexecuting instructions to process data and perform operations asdescribed herein. By way of example and not limitation, the processor(s)616 and 640 can comprise one or more Central Processing Units (CPUs),Graphics Processing Units (GPUs), or any other device or portion of adevice that processes electronic data to transform that electronic datainto other electronic data that can be stored in registers and/ormemory. In some examples, integrated circuits (e.g., ASICs, etc.), gatearrays (e.g., FPGAs, etc.), and other hardware devices can also beconsidered processors in so far as they are configured to implementencoded instructions.

The memory 618 and the memory 642 are examples of non-transitorycomputer-readable media. The memory 618 and the memory 642 can store anoperating system and one or more software applications, instructions,programs, and/or data to implement the methods described herein and thefunctions attributed to the various systems. In various implementations,the memory can be implemented using any suitable memory technology, suchas static random-access memory (SRAM), synchronous dynamic RAM (SDRAM),nonvolatile/Flash-type memory, or any other type of memory capable ofstoring information. The architectures, systems, and individual elementsdescribed herein can include many other logical, programmatic, andphysical components, of which those shown in the accompanying figuresare merely examples that are related to the discussion herein.

Although FIG. 6 is illustrated as a distributed system, in alternativeexamples, components of the vehicle 602 can be associated with thecomputing device(s) 638 and/or components of the computing device(s) 638can be associated with the vehicle 602. That is, the vehicle 602 canperform one or more of the functions associated with the computingdevice(s) 638, and vice versa. Further, aspects of the perception system622 and/or the tracking component 626 can be performed on any of thedevices discussed herein.

FIG. 7 illustrates an example processes 700 to perform techniquesdiscussed herein. The process 700 (as well as aspects of the processes300, 400, 500 described herein) are illustrated as logical flow graphs,each operation of which represents a sequence of operations that may beimplemented in hardware, software, or a combination thereof. In thecontext of software, the operations represent computer-executableinstructions stored on one or more computer-readable storage media that,when executed by one or more processors, perform the recited operations.Generally, computer-executable instructions include routines, programs,objects, components, data structures, and the like that performparticular functions or implement particular abstract data types. Theorder in which the operations are described is not intended to beconstrued as a limitation, and any number of the described operationsmay be combined in any order and/or in parallel to implement theprocesses. The process 700 may be performed by any device or component,including those of the vehicle 602. However, the vehicle 602 is notlimited to performing the operations of the process 700.

In more detail, the process 700 includes, at an operation 702,determining, using a first vehicle model, a vehicle trajectory. Forexample, the first vehicle model may be a kinematic model of a vehiclethat approximates four-wheel steering for the vehicle using mirroredsteering. The operation 702 can determine the trajectory based at leastin part on a desired path along which the vehicle is to travel and acurrent, e.g., measured, leading wheel steering angle. The operation 702may include aspects of the process 300, for example.

At an operation 704, the process 700 includes determining, based on thevehicle trajectory and a second vehicle model, one or more feedforwardcommands. For instance, the operation 704 can include determining anestimated leading wheel steering angle and an estimated trailing wheelsteering angle for executing the trajectory, based on a dynamic model ofthe vehicle. The operation 704 can include aspects of the process 400,including the operation 408, for example.

At an operation 706, the process 700 includes determining, based onmeasured errors, one or more feedback commands. For example, theoperation 706 can include determining a tracking error, e.g., one orboth of a lateral error and a heading error, and generating a frontsteering angle and a rear steering angle for correcting for theseerrors. The operation 704 can include aspects of the process 400,including the operation 422, for example.

At an operation 708, the process 700 includes determining steeringcommand angles based on the feedforward command(s) and the feedbackcommand(s). For example, the steering command angles can include aleading wheels steering angle and a trailing wheels steering angle. Theleading wheels steering angle may be the sum of the feedforward leadingsteering angle determined at the operation 704 and the feedback leadingsteering angle determined at the operation 706, and the trailing wheelssteering angle may be the sum of the feedforward leading steering angledetermined at the operation 704 and the feedback trailing steering angledetermined at the operation 706.

At an operation 710, the process 700 includes determining whether thesteering command angles meet or exceed maximus angles. For example, theleading wheels may have a first maximum steering angle and the trailingwheels may have a second maximum steering angle. The first maximumsteering angle and the second maximum steering angle may be the same ordifferent.

If, at the operation 710 it is determined that the steering commandangles do not meet or exceed the maximum angles, at an operation 712 theprocess 700 includes generating a vehicle control command including thesteering command angles. For instance, the command may cause the leadingvehicle wheels to be controlled at the leading steering angle and thetrailing steering angle determined at the operation 708. In someinstances, the leading steering angle and the trailing steering anglemay be modified to comport with preferred steering ratios, steeringrates, or the like.

Alternatively, if, at the operation 710 it is determined that thesteering command angles meet or exceed the maximum angles, at anoperation 714, the process 700 includes determining one or morealternate feedforward commands using the second vehicle model. Forexample, the operation 704 may apply the first vehicle model by assumingcertain conditions. For example, the operation 704 may assume that thevehicle has no sideslip. In contrast, the operation 714 may be determinethe feedforward commands based on a non-zero sideslip, e.g., since oneor both of the leading wheels steering angle and the trailing wheelssteering angle may be constrained to the maximum steering angle.Allowing some sideslip will allow for determination of alternatesteering angles that are both within the respective maximum steeringangles.

At an operation 716, the process 700 includes generating a vehiclecontrol command including the steering command angles. For instance, thecommand may cause the leading vehicle wheels to be controlled at aleading steering angle that is the sum of a leading steering angledetermined from the alternate feedforward commands determined at theoperation 714 and the feedback commands determined at the operation 706.The command may also cause the trailing vehicle wheels to be controlledat a trailing steering angle that is the sum of a trailing steeringangle determined from the alternate feedforward commands determined atthe operation 714 and the feedback commands determined at the operation706. In some instances, the leading steering angle and the trailingsteering angle may be modified to comport with preferred steeringratios, steering rates, or the like.

EXAMPLE CLAUSES

A. An example autonomous vehicle includes: one or more processors; andmemory storing instructions that, when executed by the one or moreprocessors, cause the autonomous vehicle to perform actions comprising:receiving a trajectory for the autonomous vehicle, the trajectory basedat least in part on a path along which the autonomous vehicle is totravel; determining a first estimated leading steering angle forsteering leading wheels of the autonomous vehicle and a first estimatedtrailing steering angle for steering trailing wheels of the autonomousvehicle, the first estimated leading steering angle and the firstestimated trailing steering angle being determined based on a vehiclemodel and having a first steady-state vehicle sideslip; determining thatat least one of the first estimated leading steering angle exceeds afirst maximum steering angle associated with the leading wheels or thefirst estimated trailing steering angle exceeds a second maximumsteering angle associated with the trailing wheels; determining a secondestimated leading steering angle for steering the leading wheels and asecond estimated trailing steering angle for steering the trailingwheels, the second estimated leading steering angle and the secondestimated trailing steering angle being determined based on the vehiclemodel and having a second steady-state vehicle sideslip greater than thefirst steady-state vehicle sideslip; controlling, based at least in parton the second estimated leading steering angle, the leading wheels; andcontrolling, based at least in part on the second estimated trailingsteering angle, the trailing wheels.

B. The autonomous vehicle of example A, the actions further comprising:determining a tracking error between a state of the autonomous vehicleand the trajectory; and determining, based at least in part on thetracking error, a leading wheels corrective steering angle for theleading wheels and a trailing wheels corrective steering angle for thetrailing wheels.

C. The autonomous vehicle of example A or example B, the actions furthercomprising at least one of: determining, based at least in part on thesecond estimated trailing steering angle and the trailing wheelscorrective steering angle, a leading wheels modification angle; ordetermining, based at least in part on the second estimated leadingsteering angle and the leading wheels corrective steering angle, atrailing wheels modification angle, wherein the at least one of theleading wheels modification angle or the trailing wheels modificationangle is determined to maintain vehicle stability during a saturationevent.

D. The autonomous vehicle of any one of example A through example C,wherein at least one of: the controlling the leading wheels comprisessteering the leading wheels according to a first commanded steeringangle comprising a sum of the second estimated leading wheels steeringangle, the leading wheels corrective steering angle, and a leadingwheels modification angle; or the controlling the trailing wheelscomprises steering the trailing wheels according to a second commandedsteering angle comprising a sum of the second estimated trailing wheelssteering angle, the trailing wheels corrective steering angle, and atrailing wheels modification angle.

E. The autonomous vehicle of any one of example A through example D,wherein: the first steady-state vehicle sideslip is zero and the secondsteady-state vehicle sideslip is non-zero, and the second steady-statesideslip is based at least in part on a first difference between afeedforward leading steering angle and the first maximum steering angleor a second difference between a feedforward trailing steering angle andthe second maximum steering angle.

F. An example method comprising: determining a first estimated steeringangle for steering first wheels of a vehicle based on a first vehiclemodel and having a first steady-state vehicle sideslip; determining asecond estimated steering angle for steering second wheels of thevehicle based on the first vehicle model and having the firststeady-state vehicle sideslip; based at least in part on the firstestimated steering angle exceeding a maximum steering angle associatedwith the first wheels, determining a third estimated steering angle forsteering the second wheels, the third estimated steering angle beingdetermined based on the vehicle model and having a second steady-statevehicle sideslip greater than the first steady-state vehicle sideslip;steering the first wheels based at least in part the maximum steeringangle; and steering the second wheels based at least in part on thethird estimated steering angle.

G. The method of example F, wherein the first steady-state vehiclesideslip is zero and the second steady-state vehicle sideslip isnon-zero.

H. The method of example F or example G, wherein the first wheels areleading wheels of the vehicle and the second wheels are trailing wheelsof the vehicle.

I. The method of any one of example F through example H, furthercomprising: receiving state information about the vehicle; determining,based on the state information, a tracking error between a current stateof the vehicle and a trajectory; and determining, based at least in parton the tracking error, one or more of a first corrective steering anglefor the first wheels or a second corrective steering angle for thesecond wheels.

J. The method of any one of example F through example I, wherein thetracking error comprises: a lateral offset of the vehicle relative to aplanned path of the vehicle; and an angular offset between a currentheading of the vehicle and the planned path.

K. The method of any one of example F through example J, furthercomprising: determining, based at least in part on the maximum steeringangle and the second corrective steering angle, a first wheelsmodification angle; or determining, based at least in part on the thirdestimated steering angle and the first corrective steering angle, asecond wheels modification angle, wherein at least one of the firstwheels modification angle or the second wheels modification angle isdetermined to maintain vehicle stability during a saturation event.

L. The method of any one of example F through example K, wherein: thecontrolling the second wheels comprises steering the second wheelsaccording to a second wheels commanded steering angle comprising a sumof the third estimated steering angle, the second wheels correctivesteering angle, and the second wheels modification angle.

M. The method of any one of example F through example L, wherein thecontrolling the first wheels comprises steering the first wheelsaccording to the maximum steering angle.

N. The method of any one of example F through example M, wherein thedetermining the first estimated steering angle and the second estimatedsteering angle is based at least in part on a ratio of the secondestimated steering angle to the first estimated steering angle.

O. The method of any one of example F through example N, wherein theratio of the second estimated steering angle to the first estimatedsteering angle comprises: a negative ratio when the vehicle istravelling below a first threshold speed; and a positive ratio when thevehicle is travelling about a second threshold speed higher than thefirst threshold speed.

P. One or more example non-transitory computer-readable media storinginstructions that, when executed by one or more processors, performactions comprising: determining a first estimated steering angle forsteering first wheels of a vehicle based on a first vehicle model andhaving a first steady-state vehicle sideslip; determining a secondestimated steering angle for steering second wheels of the vehicle basedon the first vehicle model and having the first steady-state vehiclesideslip; based at least in part on the first estimated steering angleexceeding a maximum steering angle associated with the first wheels,determining a third estimated steering angle for steering the secondwheels, the third estimated steering angle being determined based on thevehicle model and having a second steady-state vehicle sideslip greaterthan the first steady-state vehicle sideslip; steering the first wheelsbased at least in part the maximum steering angle; and steering thesecond wheels based at least in part on the third estimated steeringangle.

Q. The non-transitory computer-readable media of example P, wherein thefirst steady-state vehicle sideslip is zero and the second steady-statevehicle sideslip is non-zero.

R. The non-transitory computer-readable media of example P or example Q,the actions further comprising: receiving state information about thevehicle; determining, based on the state information, a tracking errorbetween the vehicle and a trajectory; and determining, based at least inpart on the tracking error, a first corrective steering angle for thefirst wheels and a second corrective steering angle for the secondwheels.

S. The non-transitory computer-readable media of any one of example Pthrough example R, the actions further comprising: determining, based atleast in part on the maximum steering angle and the second correctivesteering angle, a first wheels modification angle; or determining, basedat least in part on the third estimated steering angle and the firstcorrective steering angle, a second wheels modification angle, whereinat least one of the first wheels modification angle or the second wheelsmodification angle is determined to maintain vehicle stability during asaturation event.

T. The non-transitory computer-readable media of any one of example Pthrough example S, wherein: the controlling the second wheels comprisessteering the second wheels according to a second wheels commandedsteering angle comprising a sum of the third estimated steering angle,the second wheels corrective steering angle, and the second wheelsmodification angle.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described. Rather,the specific features and acts are disclosed as example forms ofimplementing the claims.

The modules described herein represent instructions that can be storedin any type of computer-readable medium and can be implemented insoftware and/or hardware. All of the methods and processes describedabove can be embodied in, and fully automated via, software code modulesand/or computer-executable instructions executed by one or morecomputers or processors, hardware, or some combination thereof. Some orall of the methods can alternatively be embodied in specialized computerhardware.

Conditional language such as, among others, “can,” “could,” “may” or“might,” unless specifically stated otherwise, are understood within thecontext to present that certain examples include, while other examplesdo not include, certain features, elements and/or steps. Thus, suchconditional language is not generally intended to imply that certainfeatures, elements and/or steps are in any way required for one or moreexamples or that one or more examples necessarily include logic fordeciding, with or without user input or prompting, whether certainfeatures, elements and/or steps are included or are to be performed inany particular example.

Conjunctive language such as the phrase “at least one of X, Y or Z,”unless specifically stated otherwise, is to be understood to presentthat an item, term, etc. can be either X, Y, or Z, or any combinationthereof, including multiples of each element. Unless explicitlydescribed as singular, “a” means singular and plural.

While one or more examples of the invention have been described, variousalterations, additions, permutations and equivalents thereof areincluded within the scope of the invention.

In the description of embodiments, reference is made to the accompanyingdrawings that form a part hereof, which show by way of illustrationspecific embodiments of the claimed subject matter. It is to beunderstood that other embodiments may be used and that changes oralterations, such as structural changes, may be made. Such embodiments,changes or alterations are not necessarily departures from the scopewith respect to the intended claimed subject matter. While the stepsherein may be presented in a certain order, in some cases the orderingmay be changed so that certain inputs are provided at different times orin a different order without changing the function of the systems andmethods described. The disclosed procedures could also be executed indifferent orders. Additionally, various computations that are hereinneed not be performed in the order disclosed, and other embodimentsusing alternative orderings of the computations could be readilyimplemented. In addition to being reordered, the computations could alsobe decomposed into sub-computations with the same results.

Any routine descriptions, elements or blocks in the flow diagramsdescribed herein and/or depicted in the attached figures should beunderstood as potentially representing modules, segments, or portions ofcode that include one or more computer-executable instructions forimplementing specific logical functions or elements in the routine.Alternate implementations are included within the scope of the examplesdescribed herein in which elements or functions can be deleted, orexecuted out of order from that shown or discussed, includingsubstantially synchronously or in reverse order, depending on thefunctionality involved as would be understood by those skilled in theart.

It should be emphasized that many variations and modifications can bemade to the above-described examples, the elements of which are to beunderstood as being among other acceptable examples. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure and protected by the following claims.

What is claimed is:
 1. An autonomous vehicle comprising: one or moreprocessors; and memory storing instructions that, when executed by theone or more processors, cause the autonomous vehicle to perform actionscomprising: receiving a trajectory for the autonomous vehicle, thetrajectory based at least in part on a path along which the autonomousvehicle is to travel; determining a first estimated leading steeringangle for steering leading wheels of the autonomous vehicle and a firstestimated trailing steering angle for steering trailing wheels of theautonomous vehicle, the first estimated leading steering angle and thefirst estimated trailing steering angle being determined based on avehicle model and having a first steady-state vehicle sideslip;determining that at least one of the first estimated leading steeringangle exceeds a first maximum steering angle associated with the leadingwheels or the first estimated trailing steering angle exceeds a secondmaximum steering angle associated with the trailing wheels; determininga second estimated leading steering angle for steering the leadingwheels and a second estimated trailing steering angle for steering thetrailing wheels, the second estimated leading steering angle and thesecond estimated trailing steering angle being determined based on thevehicle model and having a second steady-state vehicle sideslip greaterthan the first steady-state vehicle sideslip; controlling, based atleast in part on the second estimated leading steering angle, theleading wheels; and controlling, based at least in part on the secondestimated trailing steering angle, the trailing wheels.
 2. Theautonomous vehicle of claim 1, the actions further comprising:determining a tracking error between a state of the autonomous vehicleand the trajectory; and determining, based at least in part on thetracking error, a leading wheels corrective steering angle for theleading wheels and a trailing wheels corrective steering angle for thetrailing wheels.
 3. The autonomous vehicle of claim 2, the actionsfurther comprising at least one of: determining, based at least in parton the second estimated trailing steering angle and the trailing wheelscorrective steering angle, a leading wheels modification angle; ordetermining, based at least in part on the second estimated leadingsteering angle and the leading wheels corrective steering angle, atrailing wheels modification angle, wherein the at least one of theleading wheels modification angle or the trailing wheels modificationangle is determined to maintain vehicle stability during a saturationevent.
 4. The autonomous vehicle of claim 1, wherein at least one of:the controlling the leading wheels comprises steering the leading wheelsaccording to a first commanded steering angle comprising a sum of thesecond estimated leading wheels steering angle, the leading wheelscorrective steering angle, and a leading wheels modification angle; orthe controlling the trailing wheels comprises steering the trailingwheels according to a second commanded steering angle comprising a sumof the second estimated trailing wheels steering angle, the trailingwheels corrective steering angle, and a trailing wheels modificationangle.
 5. The autonomous vehicle of claim 1, wherein: the firststeady-state vehicle sideslip is zero and the second steady-statevehicle sideslip is non-zero, and the second steady-state sideslip isbased at least in part on a first difference between a feedforwardleading steering angle and the first maximum steering angle or a seconddifference between a feedforward trailing steering angle and the secondmaximum steering angle.
 6. A method comprising: determining a firstestimated steering angle for steering first wheels of a vehicle based ona first vehicle model and having a first steady-state vehicle sideslip;determining a second estimated steering angle for steering second wheelsof the vehicle based on the first vehicle model and having the firststeady-state vehicle sideslip; based at least in part on the firstestimated steering angle exceeding a maximum steering angle associatedwith the first wheels, determining a third estimated steering angle forsteering the second wheels, the third estimated steering angle beingdetermined based on the vehicle model and having a second steady-statevehicle sideslip greater than the first steady-state vehicle sideslip;steering the first wheels based at least in part the maximum steeringangle; and steering the second wheels based at least in part on thethird estimated steering angle.
 7. The method of claim 6, wherein thefirst steady-state vehicle sideslip is zero and the second steady-statevehicle sideslip is non-zero.
 8. The method of claim 6, wherein thefirst wheels are leading wheels of the vehicle and the second wheels aretrailing wheels of the vehicle.
 9. The method of claim 6, furthercomprising: receiving state information about the vehicle; determining,based on the state information, a tracking error between a current stateof the vehicle and a trajectory; and determining, based at least in parton the tracking error, one or more of a first corrective steering anglefor the first wheels or a second corrective steering angle for thesecond wheels.
 10. The method of claim 9, wherein the tracking errorcomprises: a lateral offset of the vehicle relative to a planned path ofthe vehicle; and an angular offset between a current heading of thevehicle and the planned path.
 11. The method of claim 9, furthercomprising: determining, based at least in part on the maximum steeringangle and the second corrective steering angle, a first wheelsmodification angle; or determining, based at least in part on the thirdestimated steering angle and the first corrective steering angle, asecond wheels modification angle, wherein at least one of the firstwheels modification angle or the second wheels modification angle isdetermined to maintain vehicle stability during a saturation event. 12.The method of claim 11, wherein: the controlling the second wheelscomprises steering the second wheels according to a second wheelscommanded steering angle comprising a sum of the third estimatedsteering angle, the second wheels corrective steering angle, and thesecond wheels modification angle.
 13. The method of claim 12, whereinthe controlling the first wheels comprises steering the first wheelsaccording to the maximum steering angle.
 14. The method of claim 6,wherein the determining the first estimated steering angle and thesecond estimated steering angle is based at least in part on a ratio ofthe second estimated steering angle to the first estimated steeringangle.
 15. The method of claim 14, wherein the ratio of the secondestimated steering angle to the first estimated steering anglecomprises: a negative ratio when the vehicle is travelling below a firstthreshold speed; and a positive ratio when the vehicle is travellingabout a second threshold speed higher than the first threshold speed.16. One or more non-transitory computer-readable media storinginstructions that, when executed by one or more processors, performactions comprising: determining a first estimated steering angle forsteering first wheels of a vehicle based on a first vehicle model andhaving a first steady-state vehicle sideslip; determining a secondestimated steering angle for steering second wheels of the vehicle basedon the first vehicle model and having the first steady-state vehiclesideslip; based at least in part on the first estimated steering angleexceeding a maximum steering angle associated with the first wheels,determining a third estimated steering angle for steering the secondwheels, the third estimated steering angle being determined based on thevehicle model and having a second steady-state vehicle sideslip greaterthan the first steady-state vehicle sideslip; steering the first wheelsbased at least in part the maximum steering angle; and steering thesecond wheels based at least in part on the third estimated steeringangle.
 17. The non-transitory computer-readable media of claim 16,wherein the first steady-state vehicle sideslip is zero and the secondsteady-state vehicle sideslip is non-zero.
 18. The non-transitorycomputer-readable media of claim 16, the actions further comprising:receiving state information about the vehicle; determining, based on thestate information, a tracking error between the vehicle and atrajectory; and determining, based at least in part on the trackingerror, a first corrective steering angle for the first wheels and asecond corrective steering angle for the second wheels.
 19. Thenon-transitory computer-readable media of claim 18, the actions furthercomprising: determining, based at least in part on the maximum steeringangle and the second corrective steering angle, a first wheelsmodification angle; or determining, based at least in part on the thirdestimated steering angle and the first corrective steering angle, asecond wheels modification angle, wherein at least one of the firstwheels modification angle or the second wheels modification angle isdetermined to maintain vehicle stability during a saturation event. 20.The non-transitory computer-readable media of claim 19, wherein: thecontrolling the second wheels comprises steering the second wheelsaccording to a second wheels commanded steering angle comprising a sumof the third estimated steering angle, the second wheels correctivesteering angle, and the second wheels modification angle.